Diisocyanates which are liquid at room temperature have numerous advantages over solid diisocyanates. Diisocyanates which are liquid at room temperature, such as toluene diisocyanate or hexamethylene diisocyanate, are, as a rule, physiologically harmful because of their high vapor pressure. For this reason, various attempts have been made to start with diisocyanates that are solid at room temperature and convert these into liquid form.
The most commercially important diisocyanates which are solid at room temperature are 4,4'-diphenylmethane diisocyanate and the 2,4'-isomer thereof, which melt at 39.degree. C. and 34.5.degree. C., respectively.
Numerous patents have issued relating to the liquification of diphenylmethane diisocyanate. See, for example, U.S. Pat. Nos. 3,152,162; 3,384,653; 3,394,165; 3,449,256; 3,640,966; 3,641,093; 3,674,828; 3,701,796; 3,883,571; 4,014,935; 4,055,548; 4,088,665; 4,031,026; 4,102,833; 4,115,429; 4,118,411; 4,154,752; 4,177,205; 4,229,347; 4,261,852; 4,321,333; 4,332,742; 4,490,300; 4,490,301; 4,490,302; 4,539,156; 4,539,157; and 4,539,158.
Urea group-containing polyisocyanate mixtures which are liquid at room temperature are also known. See, U.S. Pat. No. 4,611,083. Such mixtures are prepared by reacting polyoxyalkylene polyamines having functionalities of from 2 to 5 and amine numbers of from 20 to 250 with specific polyisocyanates based on diphenylmethane diisocyanate. The specific polyisocyanate used is selected from the group consisting of (i) at least one diphenylmethane diisocyanate isomer and (ii) a mixture of diphenylmethane diisocyanates and polyphenylpolymethylene polyisocyanates where the diisocyanate content of the mixture is from 55 to 90 weight percent.
It is also known that diphenylamine diisocyanates and the higher derivatives can form stable prepolymers with primary amine terminated materials having a degree of amination of from 25 to 85% and with secondary amine-terminated materials having a degree of amination of from 25 to 100%. See, U.S. Pat. No. 4,686,242.
Prepolymers are also known which are prepared by reacting amine-terminated polyethers with a variety of different isocyanates including diphenylmethane diisocyanates, See, U.S. Pat. No. 4,689,356.
It is an object of this invention to provide improved organic isocyanates which are liquid at 25.degree. C. A further object of this invention is to provide organic isocyanates which remain liquid even on prolonged storage. Still another object of this invention is to provide an improved process for preparing liquid organic isocyanates.